1. Field of the Invention
The invention relates to a process for the heat treatment of metallic workpieces in a vacuum furnace by heating the workpieces and subsequently quenching them in a coolant gas under above-atmospheric pressure and with coolant-gas circulation.
2. Description of the Prior Art
Metallic workpieces, especially tools, are hardened by heating in a furnace to the austenitizing temperature of the material and then quenched. Depending on the type of material and desired mechanical properties, baths of water, oil or molten salts are necessary for quenching. Parts of high-speed steel and other high-controlled materials can also be quenched in inert gases if these are continuously cooled and circulated.
In West German Patent Nos. 2,839,807 and 2,844,843, vacuum furnaces are described in which coolant gases for quenching are passed at high gas velocity and with pressures of up to 0.6 MPa (6 bar) over the heated workpiece charges and then through heat exchangers. The necessary high coolant-gas velocities are achieved by means of nozzles or fans. Higher quenching rates can be achieved in principle by raising the coolant-gas pressure, but the gauge pressure reached with the coolant gases used at present, such as nitrogen and argon, is only up to approximately 0.6 MPa. The application of higher pressures is limited by the power of the motor which is necessary for circulation of the compressed gases. In the use of nitrogen as the coolant gas with a pressure of 0.6 MPa gauge, the necessary motor power for a fan is higher than 100 kW. However, motors with higher powers are very bulky and expensive, and are normally unsuitable for installation in a vacuum furnace.